This invention relates to an imaging composition for providing improvement in imaging speed and image quality. More specifically, it is related to soaps having metal cations which produce images when reacted with certain dye precursors.
It is well known that dark images or visibly colored images can be formed on various substrates without the use use of carbon by methods which provide for the reaction of rosin soaps of nickel or iron with dye precursor metal chelating materials such as dithiooxamide chelating agents. These chelating agents operate to form colored complexes with the transition metal cations. By applying rosin soaps of nickel or iron to a substrate, blue-purple images can be formed on the substrate by contacting the substrate with dithiooxamide dye precursors.
A particularly useful application of rosin soaps is in the formation of "carbonless" papers. Impact or pressure-sensitive self-marking carbonless papers are well known materials which have been in commercial use for many years. Ordinarily, these papers are printed and collated into form sets for producing multiple copies. Impact on the copy sheet causes each of the remaining underlying sheets to form a mark thereon corresponding to the mark applied by machine key or stylus on the top sheet without carbon paper interleaves or carbon coatings. of course, this sequence can be carried out through a number of sheets just as if carbon paper were used. The top sheet of paper upon which the impact is immediately made usually has its back surface coated with microscopic capsules containing one of the ingredients which reacts to produce a mark. A receiver sheet, placed in contact with such back surface of the top sheet has its front surface coated with a material having a complementary component reactive with the contents of the capsules so that when capsules are ruptured by stylus or machine key pressure the contents of the ruptured capsules react with a coreactant therefor on the receiver sheet and the mark forms corresponding to the mark impressed by the stylus or machine key. In the trade, these self-marking impact transfer papers are designated by the terms CB, CFB, and CF, which stand respectively for "coated back," "coated front and back," and "coated front." Thus, the CB sheet is usually the top sheet and the one on which the impact impression is directly made; the CFB sheets are the intermediate sheets which form a mark on the front surface thereof and transmit the contents of ruptured capsules from the back surface thereof to the front of the next succeeding sheet; the CF sheet is the last sheet used which is only coated on the front surface to form an image thereon and is not coated on the back surface as no further transfer is desired. While it is customary to coat the capsules on the back surface and coat the coreactant for the capsules on the front surface, this procedure could be reversed if desired.
Yet another type of self-marking carbonless paper is referred to as a self-contained paper. This term refers to paper having the front surface treated with a coating which contains both the colorless precursor, generally in encapsulated form, and a complementary color-forming reactant. Thus, when pressure is applied, again as by a typewriter or other writing instrument, the color precursor capsule is ruptured and reacts with the surrounding complementary coreactant to form a mark.
A presently preferred class of papers is made wherein the capsule coating is comprised of capsules having a liquid fill containing an N,N'-di-substituted dithiooxamide complexing agent as a dye precursor which complexes with a metal cation, which may be included in the form of a metal salt in the coating of the sheet material, to produce a vivid image. A particular N,N'-di-organo-substituted dithiooxamide used is a combination of N,N'-di-benzyl-dithiooxamide (hereinafter sometimes referred to as DBDTO) and N,N'-bis (2-octanoyl-oxyethyl) dithiooxamide (hereinafter called DOEDTO). This material is usually present in an organic solvent such as cyclohexane within the capsule and is present in the amount of about 4% to 8% of the capsule fill.
A particularly preferred metal cation used is nickel. Nickel rosinate is often used as the active ingredient in the coating since it is substantially colorless and reacts rapidly with the dye precursor to form a blue purple image.
A shortcoming of nickel rosinate systems is the length of time required for an intense image to form after the application of pressure. Although an image formed with nickel rosinate has an acceptable optical density after a period of several minutes, users of carbonless paper generally prefer that such an image be formed in a shorter time.